Tube Amp Rebuild (and Mod)





Introduction: Tube Amp Rebuild (and Mod)

Striving for that old school sound, you purchase a 'vintage' guitar amp. But it doesn't sound quite right. Well, any amp more than 20 years old is gonna need work...

What's the deal with tube amps? Why all the fuss?

Yes, they do have a special sound, one that digital 'modeling' can't quite get right... It's actually the limitations of tubes that impart that special tone--that natural compression and smooth breakup. Sure, they keep improving solid-state amplifiers--but a look at the majority of mid-range to high-quality amps, the current models (mostly all tube, tube/ss hybrids, etc.) should convince you that it's very difficult to capture 'that sound' without tubes.

Does a 'rebuild' ruin the value of my vintage equipment?

No. Maybe. I don't know. Does changing the tubes mean the amp is not longer 'vintage?' Every old amp will need a rebuild at one point or another. If this negates it's 'vintage' status, then there is no such thing as a vintage amp! Functional amps, anyway--o.k. for nutso collectors who don't actually use 'em.... Who cares about them!

This little jem is a 1961 Kay 503A instrument (guitar, harp) amplifier. Output is in the 3-4 watt range. A nice studio, or "living room amp."

Here's a taste, and there's more video on the last step:
#1 -- Mod A (my first attempt--If you only have patience to listen to one clip, play #2):

(recognize the Trash-o-caster?)

#2 -- This is Mod B, IMHO, a much better sound:
(pickups switched from middle to middle/neck phased mix, about half way)

#3 -- Quick one added, with a Gibson Les Paul, just to show the amp has some bluesy distortion..
(pretty 'raggedy' playing on this one, but it's enough to catch the sound...)

All the videos are 'clean'--guitar and amp only, no FX.

Danger! Danger! No, really, DANGER!!!!!

A tube amp, even an unplugged tube amp, stores enough electrical energy in it's capacitors to kill you! YES, KILL YOU. Don't touch it unless you take precautions. See the section on DISCHARGE THOSE CAPS!!!!!

I'm not a tube repair technician, or an expert electrician. I'm just a hobbyist. Don't take my word, do your own research, and please be careful !

Step 1: Symptoms: How Do I Know My Amp Needs a Rebuild?

--Hum: Usually a sign of bad filter caps. Poor routing is often a cause. Adding a three-prong cord will help.

--Weak signal: Bad tubes, failing output transformer.

--No highs/loss of tone: Bad tubes, coupling or bypass caps.

--Unpleasant distortion (not the kind you want): Often, this is bad tubes. Also could be the coupling caps or cathode bypass caps. Or could be filter caps, if the signal is modulated with 60 hz noise.

--Weird noises (squeals, crackles, etc.): Probably bad tubes. Sometimes failing coupling caps.

--In each case, look to the speaker, also. A torn or worn speaker cone could be the culprit. In rare cases the output transformer may be failing.

This amp had the following problems:

--good tone for 5 minutes, then a nasty, harsh clipping distortion on louder notes (yep, bad tubes were the cause.)

--Nasty hum (filter caps and routing problems.)

--Dull, muddy tone (needed new caps and tubes.)

--The original two-prong power cord needed replacing.

Step 2: What's Involved in a Rebuild?

--A recap job
--New tubes
--Addressing safely issues

It all boils down to a lot of soldering....

Original cost: ~$100 USD.

Upgrade cost:

New tubes: ~$20
Components: ~$20-$45 (depending on what's needed.)

Parts and supplies

--New tubes

Tubes availability can be an issue. But many manufacturers continued making tubes long after they were considered obsolete. And Russian-made tubes are still produced today.

Generally, N.O.S. tubes from US or European sources are preferred (N.O.S. or "new-old stock" means: manufactured previously, but never placed in service.)

--New capacitors
--New resistors (optional)
--A terminal strip or two
--New cord/plug/fuse holder/grommet (optional)


--Soldering iron (30W--your wimpy 15W is no good here.)
--Soldering supplies--solder, desoldering tool, etc.
--needle-nose pliers
--Pop rivet tool/rivets (optional)
--Drill, dremel, etc. (optional)

--Schematics. Tacked to the inside of your amp, or online

Step 3: Buying a Vintage Amp 'project'

Before you buy a vintage amp, do your homework. Check for schematics here: schematicheaven. With any luck, you'll know loads about the amp before you even purchase it! (don't make a silly mistake and buy an old solid-state amp off ebay, thinking that every amp made in the '60s was tube!)

I'd recommend hands-on--i.e., play the amp before purchasing. But there are so many vintage amps on ebay, etc., that's not always possible.

Also, read some user reviews here: Harmony-central Amp Reviews

Things to look for in a vintage amp:

--Does it work at all? This could be a recipe for frustration--if the transformers are bad, you could be looking at a very expensive project (of course, you might have a bargain, too.)

--If you plan to rebuild yourself, simplicity is good. Are you prepared to tackle an amp with tremolo and reverb circuits, as well as a multi-stage amplifier circuit? If not, stay small.

--If modding is your goal, then point-to-point wiring is best (most vintage amps are wired this fashion, anyway.) Unlike a modern PCB, point-to-point is a rats-nest approach, where the components are simply tied together with lengths of wire. This is a good thing, and actually makes noise-related problems easier to fix.

Things to avoid:

--Unsafe design

Look for at least two transformers--every tube amplifier has an output transformer hooked to the speaker. But some old amps forgo input (power) transformers, and are wired directly (indirectly, actually, through a single cap) to the AC line voltage. This type of wiring lacks 'isolation' and is dangerous! If that one old cap fails, you're a conductor! (in the electrical, not the musical sense.) Check online schematics if you can't inspect the amp personally.

The Kay 503A is a bit of a hybrid. It has an isolation transformer--but only for the amplifier circuit, not the tube "heaters." This is not ideal, but it is safer than no isolation at all. With a bit of rewiring, it's not too bad. A separate power supply for the heaters would be the best option, however. Or an external line isolation transformer (replacing the AC heater circuit with an isolated DC supply will also reduce hum.)

--Check for blown speakers or torn cones.

--For a first-timer, avoid a large complex rebuilding project.


Seriously. Do this EVERY TIME you work on the amp. If you don't, DO NOT complain if you loose the use of your hand. DO NOT come back and haunt me if you die....

The 'filter' caps can store fatal amounts of electrical current. The caps are connected near the rectifier and are part of the power supply, and aid in converting AC to DC. In fact, they are a standard component in any power supply.

If you're completely lost, and don't understand this, DON'T MODIFY YOUR AMP. You haven't enough knowledge to work on high voltage/current circuits safely...

Several ways to discharge caps:

FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)

--Take a screwdriver or a jumper and short the capacitors leads.
--OR jumper the power amp tube plate pin to GND for a minute or so (Class A, single power tube only.)
--OR jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here...

Some, or all of these methods may result in a spark... Obviously, your flesh can act as a jumper also.

It's NEVER a good idea to touch amp circuitry when it's switched on. If you're stupid enough to do it, NEVER USE BOTH HANDS--that way, there's at least a slim chance the current won't flow through your heart.

And don't work on amps in bare feet, on a damp basement floor. :-(
Don't even play your amp in this fashion...

Step 5: General Info About 'Recapping'

ALL tube amps need a recapping job at some point. Old power supply 'filter' capacitors are the primary cause of hum. And used or not, the capacitors fail over time. Electrolytic (tubular) capacitors have an optimum life of 10 years. Of course, they may last much longer. They will often operate sub-optimally for much longer.

But tube gear is sensitive to cap failure. Tubes circuits operate at higher voltages, and many amps are quite simple. Older caps with outmoded technology ('paper electrolytic') are almost certain to fail over time. In one sense, tubes are less susceptible to cap failure--unlike solid-state amps, many will continue to operate, although poorly.

Cap types

Most common replacement caps are:

Non-polarized (polypropylene, or mylar)
Rating: 500V minimum

Most builders (and rebuilders) use high quality polypropylene ('orange drop') caps. But a quality mylar cap exceeds the characteristics of the original capacitors. I wanted to retain the vintage sound, so I choose mylar. It's amateur builders, not pros, that insist a certain type of cap is a must. In any event, the difference is subtle, and the qualitative differences are a matter of personal taste.

Is the difference between cap types real? Probably.
Here's an interesting cap bench test.

Polarized (electrolytic)
Rating: depends on the amp. I used 350V caps where the originals were 150V.

Regular 'computer' type electrolytic capacitors work fine. Especially for the power supply filter caps.

They come in two packages: radial, with the leads on the same end, and axial, with the leads on inline on both ends.

Oh, the name implies, polarized caps must be installed with the correct....polarity. Inserting one backwards--fireworks!

Note: The first set of filter caps installed were purchased at a local electronics store. This store is nerd heaven--or so I thought. Some of their components are in yellowing packaging; clearly getting old. I picked what looked like the newest of the bunch. The caps (40uF, 350V) lasted for two weeks before failing--first a bit of crackling, then a lot of crackling and the amp began cutting out....

From now on, it's mail-order from a reputable firm (digikey, jameco, etc.)

Step 6: Replacing the Filter 'can Capacitors'

Many guitar amps have the larger electrolytic caps in sealed "cans." Quite a few, this one included, have multi-capacitor cans--more than one capacitor together in a single package. They usually are the power source filter caps.

These can be replaced, but--

--Replacements are 5-10X as expensive as a handful of single caps
--Even if you can find an electrically-identical replacement, finding the same physical dimensions is difficult (height, diameter, etc.)
--Fewer options with cans. If circuit changes are made, and not all the sections are used, it's just a waste.
--You don't always know if you're getting new cans or 20 yr old 'N.O.S' ('New Old Stock'--good for tubes, bad for caps.)
--Even brand-new caps have a failure rate. Much less painful to replace one $7 cap than a $40 "can" type...

So it's quite easy and common to replace with individual polarized capacitors. I removed all the wiring from the can, then rerouted to the new caps with a terminal strip. Terminal strip are cheap, simple to use, and very common in vintage gear.

The terminal strip was attached to the chassis with a steel pop rivet and steel rivet backing-plate. The hole was pre-existing (cool!) but drilling a new one would be easy.

The old 'can' was left in place for looks, and for a reference to whoever 'recaps' this amp in the future.

The filter caps, originally rated at 150V, were replaced with 350V versions. Yes, you caught me--the 20 uF bypass cap is still only 160V, because that was the only one I had on hand (This is part of Mod A--I've since replaced it with a 10uF 350V cap...)

When replacing the filter caps, don't exceed the capacitance values by much (I went from 40 uF spec to 47 uF.) More capacitance sounds good, right? Less hum, you say? Unfortunately, amps with tube rectifiers can't handle high capacitance values--they cause voltage spikes that wear out the rectifiers quickly. 60 - 100 uF is the recommended max for this type of rectifier...stay near the spec values here.

This link has a tutorial on encapsulating (hiding) new caps inside an old cap can:

Step 7: Replacing 'the Other' Caps

Other than the filter capacitors, the other caps in the amp will be one of these (Step 8 will help you identify each of these types) :

1) Coupling Caps -- connect the signal path between stages
2) Tone Caps -- roll off different frequencies to ground, for tonal effects
3) Cathode-Bypass Caps -- integral part of "cathode biased" circuits

Most of these caps will be of the non-polarized variety. The occasional cathode bypass cap could be a polarized, however.

The coupling and tone caps should have voltage ratings comfortably higher than the max voltage the circuit might see--with cap and induction spikes, just assume these should be 400 volts or greater.

The bias voltage for tubes rarely exceeds 10-15 volts, however. A 50V capacitor will work fine for the cathode-bypass caps.

Use a 30W soldering iron. Grasp the leads of the old caps (with needle-nose pliers, if you are smart), and apply the iron. They may take some wrestling to remove. If you can avoid cutting the lead, so much the better.

Pulling vs. cutting usually leaves you a hole to insert the new replacement cap. Finagle it in, solder and you're done....

Many people would say I'm a bad boy for not using 'orange drops'....(seriously, the amp sounds much better than before, with the new mylar caps..) I do expect to be criticized for not using them, but the majority of professional amplifier technicians say that the differences are subtle and a matter of taste...

Still, remember these are the caps that the audio signal passes through. So avoid the cheapest ceramic caps, for instance.

Step 8: Mod That Amp!

A rebuild is the perfect time to mod your amp!

Here's a great link: Tubes for Dummies! :-) It outlines how each component in a tube circuit functions, and explains their purpose. Note: designing viable circuits really is more complicated than this...

Search for schematics of similar amps. I found at least 7 schematics of amps that use the 12AU6, 35Z5 and 50L6 tubes. Compare those schematics to your amp. Having similar examples is the best way to start, especially if you are unfamiliar with tube circuitry.

Note that the schematic drawing for the 12AU6 looks different--this is a pentode valve, rather than a triode, which is normally used for preamps. But the component functions are analogous.

Make one change at a time. Really large changes will probably be...unsuccessful (unless you really know what you're doing, or the design was flawed to begin with.) Here are some examples:

--Change coupling caps. They'll effect the tone. Smaller values will be brighter, larger values will add bass.

--Adjust the " grid leak resistor ," which effects the impedance and the gain of the stage.

--The initial input attenuation/RC network can be adjusted (resistors directly after the jack.)

--Cathode bypass caps on both the preamp and the power amp stage can be adjusted. This effects both the response and the tone (to a lesser extent.)

--Changing the values of the Plate Resistor and the Cathode Resistor have a direct effect on the gain, headroom and distortion. Don't do this without a thorough understanding of the tube's characteristics, however.

--Changing your speaker is another modding possibility. Be sure to match the speaker's impedance to the transformer.

--Output transformers are harder to replace, unless the schematic is well documented. If it's not, look up the power amp tube data sheet and it's 'load resistance.' For instance, load resistance for the a 50L6 @ 100V is 2000 ohms, and @ 200V, 4000 ohms. This amp uses a plate voltage of 135V, so 2500-3000 ohms for the primary is close. Choose the secondary impedance based on the speaker...

Step 9: The 'Base Mods'

These modifications are common to both Mods A & B...

--The grid leak resistor on the preamp (12AU6 tube.) Other similar designs for a 12UA6 preamp fell in a range from 2.2 Meg to 15 Meg. The Kay 503 had a 3.9 Meg resistor--I substituted a 15 Meg. This gives the preamp perhaps a bit more 'gain.'

--Input Stage: I removed the 22K resistor tied to GND here, as it wasn't wired correctly, anyway (it was wired directly to GND before the series resistor--effectively making the circuit for inputs #2 & #3 different from #1. Unless that was the plan, but it's not on the schematic....)

One input jack was removed. NO ONE EVER plugs in more than one instrument (unless they don't care about audio quality.) Two inputs jacks were plenty--each one slightly different, more room to work, and less noise. One is wired direct to the coupling cap, the other through a 22K resistor.

A one meg resistor to ground was added for higher input impedance. In this configuration, it's identical to using a one meg POT (at max) as an input volume control.

--The filter cap replacement (See the page Replacing the filter 'can capacitors.' )

--Grounded plug mod (see the page Three-prong plug safety mod.)

Now, on to the real mods....

Step 10: First Time Around--' Mod A'

After plugging in a few alternate components, I liked the setup I call Mod A.

While it has a bit more sting than the the original wiring, it's also fairly clean-sounding.

Significant changes:

--Substituted a 0.1 uf bypass cap for the preamp stage. I thought this would make the signal 'muddier,' but the extra bass added something to the sound.

--Used a 20 uF cathode bypass cap for the power amp stage. This changed the response to more treble, and together with the preamp alterations help shape the sound.

The 'Mod A' circuit has a real 'bell-like' tone.

Step 11: 'Mod B'--the Next Step

Mod A was pretty good, but I still wanted a bit more 'crunchy' from the amp. Mod B is my best effort (yet.)

Mod B has the following improvements:

--Following the example of Silvertone amps with similar tubes (1430, 1448, etc.) the first coupling cap was changed from .05 to .01uF, resulting in a good boost to the high-end.

--Along the same lines, the preamp cathode bypass cap was changed to 0.022 uF (smaller cap values here tend to lessen bass response.)

A second, larger cap (0.068uF) was added with a cut-out switch, so a fuller, jazzier tone can be added at will. The difference is subtle, but noticeable. I find it's difficult to be objective about the tonal effects of cathode bypass caps, but changes seem to alter the response, or 'tightness' of the attack, and to a lesser extent the overall though the note 'envelope' itself were subtly changed.

--The preamp-to-power-stage coupling cap was changed from .005 to .001uF. Again, more high-end boost.

--The volume control POT upped to 1 Meg (from 500K.) Since it also functions as a grid leak resistor for the power amp, this extra resistance ups the gain. Well, less signal attenuation, anyway. The additional distortion is noticeable...

Replacing with a 2 Meg POT, or moving the volume control to the preamp are other options for punching up the power stage.

--The power amp cathode bypass cap is now 10uF, further enhancing the high-end response (or actually, reducing the bass.)

Step 12: Routing, and Other Tips...


This turned out to be significant for my amp. The tone signal wire was routed beside the filament heater loop (the wire had a funny kink in it, which indicates that it was originally routed elsewhere.) The yellow wire in the photos is the heater circuit.

Signal wires should be separated from the heater circuit, especially if it's an AC loop, like this one. It's a prime source of hum.

Any long reaches, such as output to the transformer should be in twisted-pairs. A twisted-pair signal wire performs nearly as well as a fully-shielded cable.

If the hum persists, perhaps a ground-loop is at fault. Point-to-point wiring should utilize 'star grounding,' in which loops are avoided.

Oh, oh, Don't try this at home, Kids!:

One way to chase down hum is to move the components around while the amp is live. A wooden stick can be used to adjust position.

Don't use a pencil! Don't cross wires and short them! If you're smart, don't do this at all....

Other tips

--Testing with the speaker in the cabinet has a significant effect on the sound. An unenclosed speaker will have more treble (and likely not be as loud, too.)

--While on the subject of speakers, be very careful with screwdrivers, etc. around the speaker(s). Depending on the size, a new speaker will set you back $30 to $300 USD (reconeing is another option, but better to just avoid the issue.)

--Connecting two speakers in parallel reduces resistance, increasing the load. If both speakers are 4 ohm, then the load becomes 2 ohm! This is perilously close to a 'hard short.' A tube power amp can probably handle the current, but if you're messing with a transistor power amp, beware!

Even if the amp circuitry survives, there's a good chance you'll blow the output transformer if the speaker isn't close to spec...

Step 13: Three-prong Plug Safety Mod

If your amp doesn't have a grounded plug, then this mod will make it both safer and quieter. I reused a computer cord. This hasn't completely isolated the amp from line (the heater circuit mentioned in previous references), but it should be safe enough to use.

A grounded AC plug (in the USA) has the following color codes:

--BLACK : Hot
--WHITE : Neutral
--GREEN : Ground

Route the black wire through a fuse holder to the power switch. The white wire is connected directly to the transformer. Connect the green GRD to the chassis ground. This means screwed or riveted directly to the chassis--not just soldered to an existing grounded wire.

The fuse holder (and fuse) was added, not being OEM for this amp. So a 1/2 hole was drilled for mounting.

A larger rubber grommet replaced the original cord grommet. The 'stress relief' consists of two zip ties, and could be better...

Here's an in-depth explaination of the mod:

For folks outside the US, the international power cord color codes:

--BROWN : Hot
--BLUE : Neutral

Step 14: Final Thoughts

I'm very happy with the 'new' sound, and the safety improvements on the amp. It's a nice 'living room' and recording amp. The amp is brighter and a bit 'punchier' than it was originally. It has a real bell-like tone...

From the video clips, it's obvious that the amp is now very quiet from a noise perspective (well, for Mod A, anyway)--sorry, no "before" video to compare to... For an amp that's only 3 watts (perhaps a bit more with fresh tubes), it's louder than you'd think.

While the Mod A upped the treble and punch, and Mod B got some of the the 'garage' tone I was hoping for, I'm not completely satisfied... If more 'drive' is the goal, the single stage preamp would probably limit the smoothness....And if radical changes were made--well, better to just build one from scratch....

Would I do it differently next time? Yes.

I'd tear the amp down to the bare chassis and rebuild from there. But what began as a simple 'filter cap' recap job sort of snowballed...

I'd certainly clean up the interior, tidy the wiring rats-nest, remove the dust, etc. Probably go the extra mile and use the 'orange drop' caps.

Since this isn't a highly desirable amp like an old Tweed Champ, the materials used aren't going to tick-off any collectors. The design itself utilizes common radio tubes, so even N.O.S. tubes are cheap and plentiful. I can experiment to my heart's content.

Anyway, I learned something, and I'll know better next time.

More video/audio
All examples are 'clean'--just guitar and amp, no F/X.

#4 -- Mod A, again with the Trash-o-caster (the video cam doesn't quite capture the treble correctly):

Obviously, the sound quality of a small digital camera (functioning as a vid recorder) isn't going to be excellent...but it's good enough to get an idea...

Aesthetic disclaimer

Everyone's taste in sound is different. it's a personal thing. So before you write:

But my Supratronic Billy-Bob-Mcgee Boutique Amp sounds better than this!
Your mods suck, it doesn't sound like a (insert Marshall, Fender, Vox, etc. here)!
That sucks, it doesn't sound 'metal' at all!
HAHAHAHAHA, my Whooptytron Jimiaxeulator is 100000 watts! What good is this???

Well, you can't make a full-sized SUV from a go-cart chassis (but both are cool!) Any tube amp can be modified and the sound shaped, but it's basic character will not change. This is a single-stage preamp, Class A tube amplifier.

It's really a cool-sounding amp. But I understand if it's not your cup-o-tea...

And I have other amps. If I want a different sound, I'll use them.



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    I just happen to have one of those! I did some basic 40 year maintenance work on it, but I put in an old jensen speaker, and in moving the transformer from the speaker to the chassis, something went wrong and it's dead now. I think it may need a new output transformer. I benched it and figured I'd get back to it later. I'll read through this carefully when that time comes. I've fixed a lot of vintage high-fi gear. Looks good, lots of safety warnings! I've had some close calls, but never got bit. My friend was working on a tube radio at his desk when he was a kid. He touched the power supply and woke up on top of his bunkbed! No kidding folks, every time you open one of these you take your life in your hands.

    That thing sure is clean. Usually those are in terrible shape. Mine is pretty beat. The frosting drizzle grill is hilarious! Psychedelic man...

    Thanks, man! They are pretty cool...

    Always check the tubes before blaming the OT. You can check the OT too, with a VOM, just make sure neither coil is "open" or a dead short. Any reasonable low resistance value is probably OK...

    When I removed the OPT from the speaker, I had to rewire it and solder to those tiny magnet wires the coil is wrapped from. I may have failed in that, endeavor somehow but it is obvious that it was very hot at some point as the wax has oozed out of the transformer a little bit. It will probably be a while till I get back to that project. I'm much more partial to a little Gibson ranger I have anyway as far as actually playing something goes. That thing is awesome. Like a lot of those small old amps, the Kay seems more like something someone slapped together to sell than something that was created for great tone and versatility.

    Got it. Good luck.

    Yeah, most small amps don't have the tone (for me). The best small SE I've played is a Silvertone DM10. These days I prefer something with reverb, great cleans and enough wattage to project (12 to 30W is good).

    Hey gmoon. Do u have any info on an Ampeg vt60 combo. I'm trying to bring it back to life and I'm having a hard time. Thanks.

    No direct experience. I know there's a VT120 schematic on Schematic Heaven, which is probably identical to the VT60, other than having double the output tubes...

    Hey gmoon. Ur helping me a lot.

    Cool. Be safe with this stuff...

    hi! I'm thinking of modding my amp and found this very helpfull. only one question: what you mean with high-end? I thought lowering those caps would increase the bass... thanks for all